Answer
The atypical ribs are numbers 1, 2, 10, 11 and 12.
#1. This is the shortest of the ribs, and the widest. It has no costal groove, but has a small head with an articular facet that articulates with the T1 vertebra. It also has two superior grooves which support the subclavian artery, and the subclavian vein.
Rib # 2 is not very special but it is an unusually long rib.
Ribs 8 through 12 are called false ribs: they lack independent cartilaginous attachments to the sternum.
In the cases of ribs 8 -10 , their costal cartilages attach to the costal cartilage of rib # 7. Rib # 10 is also strange because it attaches to the body of a single vertebra (T10) rather than between two adjacent vertebrae as the typical ribs do.
Ribs 11 and 12 are called floating ribs. They are peculiar in that they have no attachment to the sternum (anteriorly); their pointed ventral ends are capped by cartilages which are embedded in abdominal muscles.
Rib 11 has no neck or tubercle but contains a shallow costal groove. By comparison rib 12 is shorter than rib 11 and has no costal groove.
Work Step by Step
Ribs 3-7 (are usually considered typical ribs. They have variations of the following structures: head, neck, and tubercle at the dorsal/proximal end, a curved area or angle, and a ventral/distal flat area called the shaft. A costal cartilage connects the shaft/blade to the sternum.
The head tends to be wedge-shaped and is inserted/positioned-- in the intact thoracic cage --between adjacent vertebrae. Each side of the head has a smooth articular surface : the superior articular facet joins the inferior costal facet of the vertebra above, and the inferior articular facet joins superior costal facet of the vertebra below.
The neck of a rib is the narrowed region of the rib just ventral to the head-- beyond which there is a wider rougher region called the tubercle.
The tubercle is the area of attachment between the rib and the transverse costal facet of the rib and the vertebra of the same number.
The angle. This is the concave turn that the rib makes just beyond the tubercle-- after which it proceeds towards the sternum.
The shaft. Distal/ventral to the angle the rib flattens and widens to form the shaft/blade . Each shaft of a typical vertebra has on its lower (inferior) side a depression called a costal groove. The costal groove provides a path for the intercostal blood vessels and the intercostal nerve.
